Manufacturing of electronic wafer chips involves a step wherein semiconductor work-pieces are cleaned with a liquid solution during or after Chemical Mechanical Planarization (CMP). A “semiconductor work-piece” is a microelectronic device, which has not completed the fabrication process, typically a silicon wafer with active regions formed in or on the surface of the silicon wafer. Connections to the active regions are made using multiple layers of metal, typically copper and tungsten, which has been deposited on the silicon substrate. When copper is used as the interconnect material, a damascene process is used whereby the copper is deposited into lines etched into the inter-layer dielectric and then the excess copper is removed and the surface planarized using a CMP process, followed by a cleaning step. The goal of the cleaning process (“Post-CMP cleaning”) is to remove residues left by the CMP step from the semiconductor work-piece surface without significantly etching the metal, leaving deposits on the surface, or imparting significant organic (such as carbon) contamination to the semiconductor work-piece. Furthermore, it is desirable to protect the metal surfaces from corrosion by various mechanisms such as chemical etching, galvanic corrosion or photo-induced corrosion. Corrosion of the metal surfaces results in metal recess and thinning of the metal lines. Acidic cleaning solutions are often quite efficient at removing organic contamination from the wafer surface and complexing residual copper. Thus it is desirable have a cleaning solution that is effective in the moderate to low pH regime. Acidic chemistries are typically utilized in a brush scrubber or megasonic cleaning unit for Post-CMP cleaning.
A cleaning solution may contain various chemicals that perform different functions during the cleaning process. A cleaning solution must contain a “cleaning agent.” A “cleaning agent” is the component of solution that removes residual CMP slurry particles, typically particles of metal, from the surface of the semiconductor work-piece. A cleaning solution may also contain “chelating agents,” “corrosion-inhibiting compounds,” and/or “surface-active agents.” A “chelating agent” helps prevent re-deposition of removed metal onto the semiconductor work-piece by complexing the metal in the cleaning solution. A “corrosion-inhibiting compound” is the component of the cleaning solution that protects the metal surface from attack by mechanisms such as the aggressive nature of the cleaning solution, oxidation, post cleaning corrosion, galvanic attack, or photo-induced attack. A “surface-active agent” is a component of the cleaning solution that modifies the wetting characteristics and prevents watermark formation.
U.S. Pat. Nos. 6,194,366, 6,200,947, 6,436,302, 6,492,308, 6,546,939, 6,673,757 and U.S. Patent Publication 2001/0004633 disclose information relevant to Post-CMP cleaning solutions. However, these references suffer from one or more of the disadvantages discussed below.
The optimum cleaning solution should protect the metal surfaces of the semiconductor device from having a high static etch rate and from oxidation of the metal surfaces by forming a protective film on the surface. The metal surfaces of the semiconductor work-piece are typically copper, and form the conducting paths of the semiconductor wafer. Due to the very small size of features on semiconductor wafers, the metal lines are as thin as possible while still carrying the desired electric current. Any oxidation or corrosion on the surface or recess of the metal causes thinning of the lines (dissolution) and results in poor performance or failure of the semiconductor device. Therefore, it is important to protect the metal surfaces from corrosion by forming a suitable corrosion resistant film on the surface of the metal. Some cleaning solutions available in the art do not provide a film forming agent, and thus suffer from a high static etch rate and/or high RMS value.
The cleaning solution's corrosion preventing abilities are quantified by measuring the static etch rate or the surface roughness (quantified by RMS, root mean square, value) of a metal surface that has been cleaned with the subject solution. A high static etch rate indicates dissolution of the metal surface is occurring. A high RMS value indicates a rough surface caused by attack of the metal. An effective protective film reduces the corrosion of the metal as indicated by static etch rate and RMS values after cleaning. The corrosion resistance of a cleaning solution can also be directly measured using electrochemical means known to those skilled in the art.
One preferred method of protecting the metal surface from oxidation corrosion is by passivating the metal surface after or during cleaning. Some existing acidic cleaning chemistries do not passivate the metal, resulting in corrosion during and after the cleaning step by oxidation of the metal surface.
It is also desirable to clean and protect the semiconductor surface in a single step. Some chemistries for planarizing a wafer surface include a cleaning step followed by an additional step of rinsing with water or an inhibitor solution. Some rinsing agents can leave deposits on the surface of the work-piece, thus contaminating the wafer. Adding a second step is also a drawback due to the fact that it lengthens the manufacturing process, complicates the process by having to handle more chemicals and more steps, and provides one more possible source of contamination or other quality control problems. Clearly, a process that cleans and protects the surface of the semiconductor work-piece is desirable.
The ability of the cleaning chemistry to remove residual metals and retain them in the cleaning solution is also an important characteristic of a Post-CMP cleaning solution. Chemicals that can complex the residual metals in the cleaning solution are effective cleaning solutions because the residual metals are not re-deposited on the semiconductor work-piece after they are removed. These complexing chemicals are referred to as “chelating agents.” Cleaning solutions using chemistry that cannot complex the residual metals typically perform poorly at the desired cleaning task. Thus, it is desirable to have a cleaning solution capable of removing and complexing the dissolved metal in the cleaning solution.
Another common problem with cleaning semiconductor surfaces is the deposition of contaminants on the surface of the semiconductor device. Any cleaning solutions that deposit even a few molecules of undesirable composition, such as carbon, will adversely affect the performance of the semiconductor device. Cleaning solutions that require a rinsing step can also result in depositing contaminants on the surface. Thus, it is desirable to use a cleaning chemistry that is will not leave any residue on the semiconductor surface.
It may also be desirable to have a surface wetting agent in the cleaning solution. Surface wetting agents prevent contamination of the semiconductor work-piece by helping to stop spotting of the surface caused by droplets clinging to the surface. Spotting (also called watermarks) on the surface can saturate metrology tools that measure light point defects, thus masking defects in the semiconductor work-piece.
As indicated above, the available cleaning solutions do not adequately meet all of the requirements of post-CMP cleaning. The chemistry of the current invention makes use of multiple additives to provide a solution that is not sensitive to oxygen, removes particles efficiently, removes metal from the dielectric surface, is in the neutral to low pH range, protects the metal from corrosion and dissolution, and does not contaminate the semi conductor surface.